In recent years, floating structures of various floating types such as a spar type have been proposed as offshore wind power generation floating structures. A wind power generation device installed in such floating structures includes a mast, a nacelle, and blades. A spar type floating structure is constituted, for example, by a columnar floating body and a ballast, and the floating body erected on the sea by the weight of the ballast is moored by mooring cables (see, e.g., Patent Document 1 and Patent Document 2).
For example, Patent Document 1 discloses a spar type floating structure for wind power generation on the sea, which includes upper and lower lids, a hollow lower floating body having cylindrical precast concrete blocks continuously arranged between the upper and lower lids and joined together by PC steel, a hollow upper floating body having an upper lid and precast concrete blocks which are joined to the lower floating body by PC steel and which are smaller in diameter than the above precast concrete blocks, and a ballast tank joined to the lower surface of the lower floating body by connecting steel pipes.
Floating structures such as the one disclosed in Patent Document 1 are associated with a problem that, while work such as installation or maintenance of a floating structure is performed, the floating structure rolls due to waves, wind or the like, making it difficult to carry out the work. Conventionally, therefore, the floating structure is towed by a working ship to an area where the rolling of the floating structure is small, and work such as installation or maintenance of the floating structure is performed there.
For example, in the Background Art section of Patent Document 2, a method of installing an offshore wind power generation device in a spar type floating body is described in which the spar type floating body is laid down and towed to a place of installation, ballast water is poured into the spar type floating body to erect the floating body, mooring cables connected with anchors are tied to the spar type floating body, and a post, a nacelle and blades are mounted on the spar type floating body.
Also, Patent Document 2 discloses a catamaran in which a storage section for storing a cylindrical spar type floating body in a laid state is formed between two hulls set apart from each other at an appropriate interval and fastened together by a connecting member, an accommodation section for accommodating the spar type floating body in an upright state is formed as a cut in the front part of the connecting member, winches for winding wires, by which the spar type floating body is fixed, are arranged on the hulls on both sides of the accommodation section, and an automatic position maintaining device is installed in one hull. The patent document also discloses a method of installing an offshore wind power generation device, in which a cylindrical spar type floating body in a laid state is stored in the storage section between the hulls of the catamaran, the spar type floating body is towed to a place of installation by the catamaran, a plurality of anchors are suspended from the catamaran, the spar type floating body is rotated about its front end and erected, and after a post, a nacelle and blades are mounted to the spar type floating body, the anchors are connected to the spar type floating body by mooring cables.